Among the supercritical coolants such as ethylene (C2H4), diborane (B2H6), ethane (C2H5), nitrogen oxide (N2O) and carbon dioxide (CO2) that may be used in the supercritical steam compression cycle disclosed in Japanese Examined Patent Publication No. H 7-18602 comprising, at least, a compressor, the cooling device, a means for constriction and an evaporator, carbon dioxide (CO2) is the primary coolant that is mainly utilized.
This supercritical steam compression cycle is one of the non-freon freezing cycle proposed as replacements for freon freezing cycles, and freezing cycles that use carbon dioxide in particular, are considered promising replacements for freon freezing cycles.
However, since carbon dioxide has a low critical point of approximately 31.1.degree. C., the external air temperature may exceed the critical point, especially during summer. In addition, during a freezing cycle operation, too, the high-pressure line (extending from the compressor to the means for constriction) in the freezing cycle naturally constitutes a supercritical area, and the pressure in the supercritical area where the temperature exceeds the critical point, which is determined by the density and the temperature, may exceed 20 MPa if the temperature is very high.
As described above, while it is necessary to ensure that all the components conform to specifications for withstanding super-high pressures in the freezing cycle in which the operating pressure is extremely high compared to that in a freon freezing cycle, there is a problem in that an improvement in the pressure withstanding performance will result in increases in the weight and production cost of the product. In other words, while it is desirable to use aluminum to constitute the components to achieve a reduction in the weight, the operating pressure in a heat exchanger or the like, in particular, cannot exceed 20 MPa at present in consideration of the pressure withstanding performance determined in conjunction with the heat exchanging capability and the strength.
Accordingly, a safety mechanism that discharges the coolant into the atmosphere when the high-side pressure exceeds a specific level may be provided. However, there is a problem in that the coolant released into the atmosphere must be replenished.
Addressing the problems discussed above, an object of the present invention is to provide a freezing cycle in which the high-side pressure can be reduced without having to release the coolant into the atmosphere in the event of a high-side pressure abnormality and the coolant is released into the atmosphere only in the event of a low-side pressure abnormality.